what is a filtered level in business modeler

 the documentation on demantra talks about the ability to create a filtered level by adding a extra from and where clause when configuring levels in the business modeler.

1.what does this do?

2.can we use the extra where without the extra from:

eg have a level GEO table name t_ep_site.want to prevent combinations consisting of certain t_ep_site_ep_id from displaying in worksheets.so can the extra where be        t_ep_site_ep_id <> some_t_ep_site_ep_id?

thanks